mips: boot: clean up self-extracting targets scenarios

1. All final targets like vmlinuz.{bin,ecoff,srec} etc. should reside in
   $(objtree)/arch/mips/boot, not in the root $(objtree) directory.
   The only file that should be left there is vmlinuz, similar to other
   architectures.
2. Add all the targets to $(targets) variable, so they'll be properly
   accounted by Kbuild. This also allows to remove redundant
   $(clean-files) (which were missing uzImage BTW).
3. Prefix all targets with $(obj)/$(objtree), depending on their
   locations.

Misc: fix the identation of the 'STRIP' quiet message.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Acked-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>